Chemiresistors (conductometric sensors) are traditionally used as building blocks for integrated chemical sensors (artificial olfactory systems, electronic noses). A chemiresistor is a device whose electrical resistance is modulated by molecular adsorption on its surface. The changes in resistance are proportional to the partial vapor pressure. Hence a chemiresistor converts the concentration of chemicals in the ambient air into a corresponding measurable electrical signal. A chemiresistor is constructed from a vapor-sensitive material placed between conducting leads. One of the best chemically sensitive materials ever discovered is nanoscale tin oxide (SnO2). The sensing mechanism of metal oxides is primarily based on the activation of atmospheric oxygen on the semiconductor surface. Consequently, catalytic reactions of gaseous species with oxygen sites on the surface induce charge transfer from the surface to the bulk, i.e., subsurface, which changes the electrical resistance of the device.
Most conventional chemical sensors are based on recognition of particular analytes, e.g., methane, carbon monoxide, hydrogen sulfide, etc. For complex mixtures, however, this approach is not the most reliable, since it causes frequent false alarms due to cross-sensitivity of sensors to different analytes. The advantage of using integrated sensory systems (electronic noses) is in their ability to learn the chemical signatures of interest, similar to training of canines. Unlike many other analytical techniques, an integrated sensor does not try to separate all the chemical components within a sample, but it perceives a sample as a whole, creating a global fingerprint. For example, the smell that emanates from coffee has hundreds of different chemical components, but our biological olfactory system (and the integrated sensor) simply identifies the total chemical composition as coffee. In an integrated sensor, the headspace from a sample (i.e., the gases emanating from a sample) is delivered to an array of chemical sensors. As each sensor is different in some way (usually broadly tuned to a different chemical group), each sensor's response to a sample is different. These responses can then be used to form a chemical fingerprint of a sample. The response is seen as a change in electrical properties (normally resistance) of the sensor. Specialized software then identifies the sample from this fingerprint.
These sensors usually suffer more or less from cross sensitivity, i.e., apart from their response to a particular target gas they do (to a certain extent) respond to other gases as well. For instance, a methane sensor is also responsive to propane, butane, and natural gas in general. In this respect, a single output sensor cannot be sufficient, even if only one target gas is to be detected. However, a combination of several gas sensors, each providing a different sensitivity spectrum, a so-called sensor array, delivers signal patterns characteristic for the gases to which the array is exposed. These signal patterns enable the distinction between individual gases or gas ensembles. Which gases can or cannot be detected or distinguished depends on the sensor type and the extent of the difference in selectivity between the sensors. Now, the cross sensitivity of the individual sensors, due to a low selectivity even turns out to be an advantage. A low selectivity (in the case of a single sensor a disadvantage for detecting a particular gas), now allows the array to respond to a wide range of gases. A combination of several chemiresistors, each providing a different sensitivity spectrum, a so-called sensor array, delivers signal patterns characteristic for the gases to which the array is exposed.
For more than two decades now, small and simple gas sensors, which provide one output signal only, have been commercially available. Typically, they are manufactured by the sol-gel method, in which metal oxide layers are deposited in the form of a viscous paste and then baked in an inert environment, creating thick films. Metal oxide sensors from Figaro (TGS sensors) and Henan Hanwei Electronics Co., ltd. (MQ sensors) are manufactured by this method. The first integrated sensory systems equipped with arrays assembled from separate sensors were manufactures in the early 90s.
Individually manufactured sensors equipped with sockets are placed in plugs on a carrier plate of several centimeters in size. There are multiple drawbacks associated with this conventional design:                1. The sol-gel process utilized for manufacturing of individual sensors does not provide precise control over the oxide layer thickness. Because of that, variations from sensor to sensor in this manufacturing process are unavoidable. As a consequence, even if the datasheet provides a calibration curve, every sensor manufactured by the sol-gel method requires a calibration and verification by the consumer, using costly specially prepared gaseous mixtures.        2. Individual sensors in a sensor array evolve over time. This phenomenon is known as a long-term drift. For a conventional integrated system, individual elements evolve differently, causing failures of pattern recognition algorithms.        3. Short-term drift due to the fluctuations in the environment also has different effects on individual elements in a conventional integrated system and causes failures of pattern recognition algorithms.        4. Frequently, individual sensors in a conventional integrated system have variances in response time. This means that some of them respond to analyte exposure faster than the others. Upon exposure to analyte but before reaching a stationary state, sensors of integrated system go through the transient phase. If they are not well-synchronized, during the transient phase, the conventional integrated system typically reports several false results. Synchronization of individual elements of a conventional integrated system is another time consuming process, and has to be implemented for each unit after the assembly.        5. If one of the sensors in a conventional integrated system fails and needs to be replaced, the entire system will need to undergo synchronization and calibration.        6. The thickness of the metal oxide layer is a key parameter that determines sensor sensitivity. The thinner the layer—the higher the sensitivity. Since only the thick films can be produced by the sol-gel method, sensors formed with this method have limited sensitivity. For most chemical compounds, the sensitivity of a sol-gel sensor is unable to go below 1 ppm.        7. Sol-gel films, which are thick, have relatively long time of recovery after exposure, which can be up to 1 min for exposure to high concentrations.        8. Conventional integrated sensory systems are typically large in size. Discrimination power of an integrated sensor depends on the number of individual basic sensing elements with different chemiresistive properties. However, an increase in the number of sensors inevitably leads to an increase in size, which causes makes a non-uniform distribution of chemicals over the sensor array upon exposure to gaseous analyte.        9. As a consequence of their size, conventional integrated systems often require sophisticated gas sampling systems, splitting the analyte gas into identical fractions for each sensor.        10. Conventional integrated sensory systems typically have high power consumption (hundreds of watts).        11. Conventional integrated sensory systems typically have high manufacturing costs, especially in the case of an advanced sampling system. Synchronization and calibration also makes the manufacturing process time-consuming.        
In view of the above, it would be advantageous to develop new chemiresistors and integrated chemical sensors and methods of making and using the same, which overcome at least some of the above-noted drawbacks with conventional integrated sensory systems.